1. Technical Field
This invention relates to the art of AM radio broadcasting and, more particularly, to a system having distributed modulation encoding for controlling the turn on of a plurality of power amplifier modules.
2. Description of the Prior Art
The H. I. Swanson U.S. Pat. No. 4,580,111 discloses an amplitude modulator for AM radio broadcasting wherein the modulator serves to generate amplitude modulated signals by selectively turning on and off a plurality of RF amplifier modules to produce amplitude modulation. The aforesaid Swanson patent has an encoder that supplies turn on commands to selected amplifier modules. In a system having a large number of power amplifiers, the power amplifiers are divided into groups with each group being provided with turn on signals from an associated modulation encoder. If, for example, each group includes 32 amplifier modules then the first modulation encoder will provide turn on module commands 0-31 if the module command is for turning on a number greater than 31. A second modulation encoder would then respond to modulation commands 32-63 and turn on all its associated power amplifier modules and so on. Any additional modulation encoders will operate with its associated group of power amplifier modules within the corresponding range of module commands.
The difficulty with the multiple modulation encoder system is that if a power amplifier module or modulation encoder fails, no reassignment takes place. Thus, if one of the power amplifiers in a first group controlled by the first modulation encoder fails, and if the failed module was supposed to be turned on, there will be an error in the output signal. There is no sharing of powering the output load among the various groups of power amplifiers in such a system. If the modulation encoder associated with the first group of power amplifier modules fails, the first group of power amplifier modules will not be turned on. Only a simple turn on scheme can be implemented, such as incrementing or decrementing the turn on of the modules upward or downward.